The N-vinylamide polymers are useful as a raw material of polyvinyl amine which has been extensively used as a coagulant, a paper-making reagent, a fiber-treating agent, an additive for paints, etc., and it has been required that they are provided in the form of polymers having various extensive molecular weights including not only a high-molecular weight polymer having a molecular weight of not less than 4,000,000 but also a low-molecular weight polymer having a molecular weight of not more than 100,000 according to the applications thereof. The N-vinylamide polymers have been conventionally produced by various polymerization methods such as a reversed phase suspension polymerization method, an emulsion polymerization method, a belt photopolymerization method (photo-belt polymerization method), an aqueous solution standing adiabatic polymerization method and an aqueous solution dropping polymerization method. Among these polymerization methods, the belt photopolymerization and the aqueous solution standing adiabatic polymerization method are more simple and convenient and have such an advantage that polymers having various molecular weights are respectively produced by appropriately selecting the polymerization conditions.
As the polymerization method for producing the N-vinylamide polymers, there are conventionally known, for example, an aqueous solution polymerization method (Patent Document 1), an aqueous solution adiabatic polymerization method using combination of a redox initiator and an azo initiator (Patent Document 2), and the like. Also, it is known that the N-vinylamide polymers are produced by a belt photopolymerization method (Patent Document 3).
Further, there is also known a method for producing a dispersion of polymer particles by copolymerizing a monomer mixture comprising N-vinylamide and acrylonitrile in which an inorganic halide is added as a particle size modifier (Patent Document 4). However, this method is not an aqueous solution polymerization but relates to the technique for producing the dispersion of polymer particles, i.e., a method of controlling a particle diameter of the polymer particles precipitated by addition of the inorganic halide. Thus, the method described in Patent Document 4 does not relate to a method of improving a gel quality of the resulting polymers and controlling a polymerization reaction rate.
Patent Document 1: Japanese Patent Application Laid-Open (KOKAI) No. 61-118406
Patent Document 2: PCT Pamphlet WO 00/58378
Patent Document 3: Japanese Patent No. 3704660
Patent Document 4: Japanese Patent No. 3216070